Testing the Acidity of Milk and Cream. 123 



more alkali should be added after the first appearance 

 of a uniform pink color in the sample. This color will 

 fade and gradually disappear again on standing, owing 

 to the effect of the carbonic acid of the air, to which 

 phenolphtalein is very sensitive. The amount of the 

 alkali solution used for the test is then obtained from 

 the reading on the scale of the burette. The per cent, 

 of acid in the sample is calculated by multiplying the 

 number of cc. of alkali solution used, by .009 and di- 

 viding the product by the number of cc. of the sample 

 tested, the quotient being multiplied by 100. 



c. c. alkali X. 009 



Per cent. acidity= .X100 



c. c. sample tested 



If 50 cc. of cream required 32 cc. of alkali solution to 

 produce a permanent pink color, the per cent, of acid in 



the cream woulft be 32 * Q9 X10Q=.58 per cent. A 



part of this calculation may be saved by using a factor 

 for multiplying the number of cc. of alkali added in 

 each test. This factor is obtained by dividing .009 (the 

 number of grams of lactic acid neutralized by one cc. 

 of alkali solution) by the number of cc. of sample 

 tested, and multiplying the quotient by 100. If a 50 

 cc. pipette is used for measuring the sample to be tested, 

 the factor will be (.009-=-50)XlOO=.018; if a 25 cc. 

 pipette is used, the factor will be (.009-=-25) X 100= 

 .036: and if a 20 cc. pipette is used, (.009-1-20) X 100= 

 .045 will be the factor to be applied in calculating the 

 per cent, of acidity, the number of cc. of alkali used 

 being in all cases multiplied by the particular factor 

 corresponding to the volume of the sample tested. 



